This invention relates to plenum drying hoppers for granular resin materials used in plastic molding and similar processing. More particularly, the invention concerns a plenum drying hopper which uses relatively high temperature heaters in the hopper itself in a construction that precludes melting of the granular material, especially fine particles of that material.
The need for higher operating temperatures in plenum drying hoppers has long been recognized, but problems have existed due to melting of the granular plastic materials, especially the fine particles or dust from these materials, when high temperature heaters are used. For other granular materials, radiant heat from high-temperature heaters may damage the material without actual melting. Another problem has existed in that fine particles which pass through the diffuser screen may fall on the heaters and must be cleaned out from the bottom of the hopper. Any fine particles or dust that falls on the heaters is likely to melt and hence is not easily removable, creating a difficult clean-up problem.